This proposal addresses a fundamental question in biology. How is complex organogenesis regulated? Studies in plant and animal systems have show that signaling centers within the developing organism play large roles in control of cell division and differentiation. Recently the SHORTROOT (SHR) protein, a putative transcription factor, has been shown to be an organizing signal in patterning of the Arabidopsis root. SHR is required for division of particular initial cells (stem cells) within the root apex, activation of SCARECROW and differentiation of the endodermal cell layer. Analysis of the patterns of SHR protein and mRNA expression indicates that the SHR protein moves in a highly specific manner from the internal cell layers of the root where SHR mRNA is expressed into the adjacent cell layer. Using transgenic plants, structure/function analysis of SHR is proposed to identify regions of the SHR protein that are required for its movement and for activation of downstream genes. Information gained from these experiments will be used to design screens for proteins involved in SHR movement and to identify SHR regulated genes.